Before 13 yrs when I had my first cocker,I got her when she was 2 months old,she was undocked at that time.We (mom & I) went to the vet and he said that we can dock her tail if we want to,and we did that.She wasn't suffering too long,only for 2 days,after that everthing was great with her.
One friend of mine did the same,she docked tail to her puppy when he was about 3 months old,nothing bad happened.
In my house,we have 2 docked cockers and 3 undocked,I prefer docked cockers,but now in our state docking is banned.I don't agree with this,but it's not on me to make a low.
Also,when I had litters,I docked their tails when they were 2 days old..
Iva